A nonlinear optical device for converting an optical frequency by applying nonlinear interaction, e.g., second harmonic generation, of an optical wave in a nonlinear optical material, in many cases, uses two concave reflection mirrors for laser-resonating. One of these reflection mirrors is a total reflection mirror (or a high reflection mirror) while the other is a reflection mirror through which a portion of laser beam transmits.
The high reflection mirror among them comprises, as shown in, for example, FIG. 4, plural reflection multilayer films 2 and 3 consisting of a plurality of reflection films 2a, 2b, 3a, and 3b laminated on one surface of a transparent substrate 1, each of which multilayer films 2 and 3 corresponds to each wavelength of plural wavelengths of a light beam to selectively-reflect one of said plurality of wavelengths, respectively. Each of the reflection multilayer films 2 and 3 is formed by alternately laminating usually different dielectric materials (e.g., SiO.sub.2 and TiO.sub.2), as shown herein as the reflection films 2a and 2b or 3a and 3b, at such a film thickness as being capable of providing a maximum reflectance at each specific wavelength.
However, in the conventional optical mirror for use in a plurality of wavelengths, as shown in FIG. 4, there is considered only a reflection amplitude, so that a phase matching condition required for nonlinear optical materials is not satisfied. As a result, when the second harmonic generation, the optical mixing or the like is performed using the nonlinear optical material, there occurs a disadvantage of decreasing efficiency due to a phase shift between an exciting light beam and a newly generated light beam when these light beams are reflected by a mirror.